The subject matter disclosed herein relates to measuring the load in a material handling system. More specifically, the subject matter disclosed herein relates to a method and apparatus for measuring the load in a material handling system using one or more motor drives to lift a load, where each motor drive controls operation of a hoist motor.
Material handling systems are widely used to lift heavy loads, weighing up to hundreds of tons. A typical material handling system includes at least one motor used to raise and lower the load and at least one additional motor to position the material handling system over the load to be moved. Common applications include manufacturing facilities, in which large components may be positioned for assembly and/or the final assembly may be moved for shipping. Further, material handling systems are often required to handle loads of varying sizes. It is known that electric motors are capable of producing rated torque up to rated speed. Therefore, it may be desirable to verify that the weight of the load the system is attempting to move is less than or equal to the rated capacity of the system, such that the operator may safely raise/lower and position the load.
It is known that this weight measurement may be performed by the addition of an external weighing device, such as a load cell. However, external weighing devices have not been realized without certain disadvantages. External weighing devices add expense in both material and commissioning costs. They load cell is typically mounted to a mechanical component of the material handling system that is subject to a degree of strain or deflection as a result of lifting the load. Consequently, the load cell may need to be mounted on the hook or another movable member of the material handling system. The feedback signal from the load cell must then be wired or wirelessly communicated to a controller. The feedback signal may also need calibration to the load.
As is understood in the art, weighing devices measure forces applied to the device. The force applied as a result of gravity acting on the load corresponds to the weight of the object; however, in a material handling system, other forces are similarly applied to the weighing device. For example, the torque required to accelerate a load when raising the load or to decelerate a load when lowering the load provides an additive force on the weighing device. Similarly, deceleration when raising or acceleration when lowering is assisted by gravity, resulting in a reduced weight being measured by the weighing device. In order to obtain an accurate measurement of the weight of the load, therefore, the motor either raises and suspends the load at zero speed or operates for a short period at a constant speed when measuring the load.
Other systems have been introduced to obtain a measurement of the load as a function of the current provided to the motor. However, these systems have similarly not been realized without certain disadvantages. Although the current provided to the motor is a function of the weight of the load, the current is also a function of the torque applied to the load. Therefore, the magnitude of the current also varies during acceleration or deceleration of the motor. Consequently, current measurement systems may also require the motor to either raise and suspend the load at zero speed or operate for a short period at a constant speed when measuring the load. As a result of the limitations on obtaining accurate measurements, systems that employ either a load cell or current measurement may perform an initial test of the weight of the load, but make no further checks as a load is raised or lowered.
However, in some applications, the weight of the load may vary as the load is raised or lowered. In some instances, the variation in weight may be unexpected. For example, in the shipping industry, containers are loaded and unloaded between storage facilities, trucks, trains, and ships. The containers are often stacked several layers high and in close proximity to each other or to walls, for example, within a hold of a ship. As the container is raised or lowered, it may sway, for example, due to the wind, or the other containers may move, for example, as the deck of a ship moves. In either instance, the container may contact or become snagged on an adjacent container, causing a change in the weight of the load. In other instances, the variation in weight is part of the application requirements. For example, bulk materials such as sand or salt may require a bucket, or clam shell, attachment. The clam-shell is lowered in an open position and closed to scoop up a load of the bulk material. As the operator begins to raise the clam-shell, there may be some initial slack in the cable. The weight of the load may be significantly lower or even near zero during the period at which the load is measured. The load may, therefore, be initially determined as safe but may subsequently exceed the rated capacity of the material handling system.
Thus, it is desirable to provide a system configured to periodically monitor the weight of the load throughout a run without requiring external load measurement devices.
In still other applications, a load may require multiple hoists for lifting due, for example, to the weight and/or size of the load. In one application, a single bridge may include two or more hoists mounted to a single bridge. In another application, multiple bridges may be included on a single set of rails and each bridge includes a single hoist. In either application, multiple hoists may be utilized to lift the single load.
When multiple hoists are used for lifting a load, the overall system load ratings must be considered. Although each hoist has a rated capacity, the bridge and rails also have load ratings which cannot be exceeded. When lifting a load with multiple hoists, each hoist may be operating within its rated capacity however the combined weight of the load on a single bridge or on a set of rails may exceed the load rating for the bridge or rails.
Thus, it is desirable to provide a system to determine a total weight of a load when multiple hoists are used to lift the load and to compare the total weight to the ratings of the bridge and/or rails on which the hoists are located.